On the photo-sharing app Instagram, search the keywords #Fairfax, #Rockville or #DC and up pops hundreds of photos from children. Among them, until recently, were many from Kyle, a 12-year-old. His full name, Gaithersburg middle school and favorite Montgomery County hangouts were on public display before his parents put a stop to it.

Technically, Kyle was not supposed to be on Instagram, the mobile app owned by Facebook. The company’s policy sets the minimum age at 13. But Kyle said he was able to join easily, no questions asked. Within minutes of setting up his account this past fall, he was uploading “selfies” of his cherubic face and blond mop top and tagging photos of friends with their names.

The photo-sharing service is experiencing tremendous growth, doubling in size to 100 million users in about a year. But child advocates and some parents say that too much of its rise has been driven by preteens or even younger children.

These advocates say they worry about whether Instagram is collecting the personal information of young children and whether the company is doing enough to make sure kids are safe from adult strangers.

Over the past two weeks, more than 4,500 people have signed a petition on Change.org that calls for Facebook to automatically set the accounts of most teens to private. It also asks the company to disable GPS technology that can pinpoint where children take photos.

“Facebook is not doing enough to ensure children under 13 don’t have access to the site,” said Joy Spencer, a director of child safety for the Center for Digital Democracy, a public interest group. “That raises a number of concerns about safety and because Instagram then is able to collect personally identifiable information on children, which can be used to target ads toward them in the future.”

Facebook’s main site, which has about 1 billion users, tries to discourage minors from joining. It requires a real name, an age and a few other bits of information when a person signs up. The gateway is hardly foolproof, because it relies on voluntary answers. Still, people who volunteer that they are younger than 13 are automatically barred from the site.

New research shows that Instagram has become particularly popular among youths. But the service does not make even a nominal effort to keep young children from signing up, privacy advocates said. Anyone can register with a fake name. The app does not ask for any personal information.

If Instagram were to ask the age of its users, it would have to take on more legal obligations, privacy analysts say.

Under a revision of the federal government’s child privacy law, set to take effect July 1, social networks and other Web sites must get a parent’s consent if they collect personal information such as photos, e-mail addresses or videos from users younger than 13.

Instagram said it does not track how many underage users are on its service, which primarily serves mobile-phone and tablet owners. When asked why, spokeswoman Nicky Jackson Colaco said, “Like many other platforms, Instagram only asks for data that is essential to operate our service.”

Colaco said Instagram is trying to make the app safer. Instagram has “dedicated reporting channels so that people can report underage activity to us . . . and we will continue to invest heavily in tools and education that help people have a safe experience on our site,” she said. Instagram, which was founded in 2010, so far has been free of ads.

Facebook and other social-media sites have said that authenticating age is difficult, even with technology. A Consumer Reports survey in 2011 estimated that 7 million preteens are on Facebook.

Other popular social-media companies, such as Tumblr, Snapchat and Twitter, have similar policies banning preteens, but few are successful in keeping young children off their services, privacy advocates say.

Photo-sharing sites pose particular risks to minors, they add.

Rebecca Levey, a blogger who writes about parenting, said the federal government is too focused on how companies advertise to youths. Parents are most concerned about whether predators have access to their children on social-media sites, she said, or what social pressures their kids may face online.

“Parents care most about what their children are sharing and who they are talking to online,” said Levey, who founded a social-media site called KidzVuz for “tweens,” children ages 10 to 12. “There are no barriers to entry for some of the most popular sites for tweens, like Instagram and Snapchat.”

Kyle’s mother, Lisa, was surprised to learn that her son was too young to join Instagram. The rule is buried deep in the terms-of-service agreement. Many of her fourth-grade daughter’s friends had accounts, as well as all of Kyle’s middle school friends. So she gave him permission to sign up, too.

Kyle checks the app daily. For more than a year, he kept his account public, which allowed him to meet new friends outside school and allowed strangers to post comments on his photos.

“I’m making friends and finding funny jokes and pictures,” he said.

After a call with The Washington Post, Lisa recently pressed Kyle to change his Instagram settings to make his photos private. Kyle’s mother spoke on the condition that The Post not use her family’s last name, out of concern for her children’s privacy and safety.

Lisa also shut down her 10-year-old daughter’s account after a friend posted a nasty remark about one of her pictures.

“That was tough,” Lisa said. “We both decided she’s not ready for it. She’s just too young at this point.”

There isn’t precise independent research that tracks the number of preteen users on Instagram. But teenagers rated Instagram as the third-most-popular social network in a joint study to be released this month by the Pew Research Center’s Internet & American Life Project and Harvard University’s Berkman Center for Internet & Society.

That surprised researchers because Instagram was not technically defined as a social network; it was mentioned voluntarily by the teens.

“We heard from teens that Instagram and other apps like it are simpler than other social-media sites like Facebook,” said Amanda Lenhart, a Pew researcher. “There aren’t as many adults on it. It’s easy to manage and fun.”

Those qualities prompted Paulina, a 12-year-old from San Francisco, to download the app last year. She describes it as “addictive.”

Recently, she sat in the back seat of a car with friends, and all three were hovered over their smartphones, posting pictures on Instagram, laughing about the comments they were racking up and comparing the number of “likes” they were accumulating.

“It’s like a scorecard for popularity, and that is a downside,” said Paulina’s mother, Courtney, who also asked that her family’s last name be withheld. “It’s the new social currency.”

Paul Miller, like many parents, helped his daughters, 11 and 13, create accounts. The tech and media consultant, who is based in San Francisco, was not aware of the age limit. He agreed to his daughters’ pleas to download the app because he wanted to encourage their use of technology and their budding interest in photography.

He quickly learned that his children needed a basic primer on etiquette and behavior. No posting photos that could make others feel excluded. No nasty comments. Say “no” to strangers.

He said he hopes Instagram will take a more active role in educating children on best practices.

“It’s a tender age, and the social etiquette isn’t there for this technology,” he said.

Sign up today to receive #thecircuit, a daily roundup of the latest tech policy news from Washington and how it is shaping business, entertainment and science.